Generally, when a concrete structure is exposed to high temperature, such as in a fire or the like, for a long time, since hardened cement bodies and aggregates show different expansion and shrinkage behaviors, cracks appear or the structure is weakened, and physical properties and fire resistance are noticeably degraded. At this time, a change in pore structure and a chemical change are made, and thermal stress generated by confinement of an end part or the like may cause cracks which causes concrete deterioration and spalling.
Specifically, a hardened cement body has a large amount of chemically-bonded water in addition to free water, and when the hardened body is exposed to 100° C. or higher, free water existing in capillary pores thereof evaporates, and a volume expansion of 1,300 times or more occurs. Also, an internal structure of a paste is loosened, which leads to an increase in pore volume and the development of cracks.
Also, when a heating temperature further rises to about 180° C., a part of the chemically-bonded water begins to evaporate from the hardened cement body. About 20% of the water content of a calcium-silicate hydration product, which is a core hydrate for solidity of the hardened cement body, is lost at a range of about 250° C. to about 350° C., and most of the water content is lost at a range of about 400° C. to 700° C. Within a similar temperature range, calcium hydroxide (Ca(OH)2), which is a free alkali component in concrete, is also pyrolyzed into calcium oxide and water and chemically damaged. Also, since the number of slightly large pores increase and hardness is lost, the hardened cement body becomes structurally very dangerous. Subsequently, when the concrete is heated at about 1200° C. or higher for a long time, the concrete melts from a surface of the structure.
FIG. 1 is a diagram exemplifying compressive strength of a concrete structure being lowered with an increase in temperature during a fire.
As shown in FIG. 1, cement hydrates in concrete of the concrete structure undergo a chemical change in accordance to an increase in heating temperature, and a cement paste and an aggregate show contrary behaviors, that is, shrinkage and expansion, respectively, at temperatures up to about 600° C. Further, as a result of the expansion of free water or the like existing in concrete capillary pores, internal stress gradually increases, and an internal structure is destroyed. Therefore, mechanical properties, such as solidity, elasticity, etc., are degraded. This is mainly because internal destruction resulting from a difference in a thermal expansion coefficient between the cement paste and the aggregate according to quality characteristics has influence on the mechanical properties of the concrete.
Here, the degree of degradation varies according to types, proportions, material ages, etc. of used materials and exhibits a tendency shown in FIG. 1. In other words, solidity is barely degraded up to 300° C., but becomes 50% or less above 500° C. Also, at about 700° C., solidity may be degraded to be in a range of about 60% to 80% of room-temperature compressive strength. Accordingly, it can be seen that compressive strength of concrete is noticeably degraded when the concrete is heated to a high temperature by a fire. Also, it can be seen that an elastic modulus is lowered by heat and is almost halved at 500° C. This is because concrete lose elasticity and gradually becomes plastic when the concrete is heated to a high temperature.
Meanwhile, when a fire occurs, such a concrete structure is degraded in performance due to a change in a microstructure thereof. Such a change in microstructure can be identified from a pore distribution or pore structure characteristics using a gas adsorption method of adsorbing nitrogen onto a concrete structure at nitrogen's boiling point (−195.8° C.) to measure a pore structure thereof.
To determine whether to reuse a fire-damaged concrete structure and a damage level thereof on the basis of a fire mechanism in such a concrete structure, it is necessary to accurately diagnose performance degradation of the concrete structure.
However, up to now, there has been neither an expert nor technology for logically describing the degree of fire damage to a concrete structure in which a fire has occurred. In other words, since there has thus far been no technology for diagnosing a fire-damaged concrete structure, an appropriate assessment is not being made. Accordingly, it is necessary to develop an assessment tool for predicting a residual lifespan of a fire-damaged concrete structure.